1. Field of the Invention
The invention relates to coherent detection methods for phase-shifting-keying and apparatus for performing such methods.
2. Discussion of the Related Art
Quadrature phase-shift-keying (QPSK) is a more bandwidth-efficient modulation scheme than simple on-off modulation schemes. In wireline and wireless communication systems, QPSK receivers typically have an analog phase lock loop (PLL) for providing the type of coherent detection typically needed to implement a QPSK modulation scheme. The PLL provides feedback that enables phase and frequency tracking of the transmitted optical carrier by a local oscillator. The local oscillator provides a reference signal for down mixing of the modulated carrier wave received from the transmitter.
In optical communication systems, schemes using an optical PLL have been proposed for implementing coherent detection. In such schemes, the optical receiver has a local optical oscillator for down mixing the received modulated optical carrier and also has an optical PLL. The optical PLL feeds back information for phase and frequency synchronizing the local optical oscillator to the optical carrier.
Unfortunately, the construction of optical PLLs is often difficult to achieve. For example, conventional laser sources often have large line widths. The large line widths can make optical phase synchronization difficult at data transmission rates. Indeed, an absence of efficient and inexpensive apparatus for implementing such synchronization has hindered the development of coherent optical communications systems that could otherwise more efficiently use the available optical bandwidth.